Field of the Invention
One disclosed aspect of the embodiments relates to an image capturing apparatus and a control method thereof, and particularly relates to a focus detection technique.
Description of the Related Art
Conventionally, in automatic focus detection and automatic focus adjustment of an image capturing apparatus, a contrast detection method is known as a general method using a light flux which has passed through a shooting lens. In this method, an image sensor is used as a focus detection sensor, and focus adjustment is enabled by evaluating an output signal of the image sensor and moving a position of the shooting lens so that contrast information thereof indicates a maximum.
However, it takes time since it is necessary to evaluate the contrast information every time the shooting lens is moved, and the shooting lens is moved again to a position at which contrast is the maximum after it is found, as a result of the evaluation, that the contrast is the maximum. Accordingly, a high-speed operation is difficult.
In order to improve such a defect, proposed is a technique with which, by including a phase difference detecting function in an image sensor, a defocus amount of a shooting lens is able to be directly obtained while the image sensor is used as a focus detection element.
For example, in Japanese Patent Laid-Open No. 2010-219958, a pupil-dividing function is provided by offsetting a sensitive region of a light-receiving portion with respect to an optical axis of an on-chip micro lens in some light-receiving elements in an image sensor. Then, by arranging pixels, which have the light-receiving elements, at predetermined intervals in the image sensor, the phase difference detecting function is realized.
In addition, for example, in Japanese Patent Laid-Open No. 2013-106194, a plurality of photoelectric conversion elements of an A pixel and a B pixel are provided in each pixel corresponding to one of micro lenses of an image sensor and an A pixel output and an A+B pixel output are read out. By performing subtraction processing of these two outputs, a B pixel output is obtained, and the phase difference detecting function is realized.
In Japanese Patent Laid-Open No. 2010-219958, disclosed is an idea that, only in a row in which a phase difference detection pixel is arranged, a reset line of so-called rolling shutter driving is independently scanned in each of a row of a normal pixel and a row including the phase difference detection pixel. In this case, a readout method of a signal in an inside of the image sensor is not different between the phase difference detection pixel and the normal pixel which are arranged in the same row. However, in Japanese Patent Laid-Open No. 2010-219958, the phase difference detection pixel is not allowed to be used as the normal pixel, so that it is difficult to extremely increase a proportion of arrangement of the phase difference detection pixel. Thus, resolution for phase difference detection is lowered.
On the other hand, in Japanese Patent Laid-Open No. 2013-106194, by forming all of pixels with the plurality of photoelectric conversion elements, all of the pixels become usable as phase difference detection elements. Therefore, resolution in a horizontal direction is also improved. However, since an A+B signal and an A signal are read out from the plurality of photoelectric conversion elements of all of the pixels, the image sensor requires a readout circuit by which each of A+B pixels and A pixels is read out. Particularly, since twice as much as a holding capacitance, which requires a relatively large area in the image sensor, is required and control lines become complicated, it is feared that a circuit area in a peripheral part of the image sensor is increased.
In addition, as described in Japanese Patent Laid-Open No. 2013-106194, in the image sensor in which all of imaging pixels of the image sensor are formed with a plurality of photoelectric conversion element units, a circuitry in a peripheral readout circuit becomes complicated, and, particularly, a circuit element having a relatively large area, such as the holding capacitance, is to be required.
Furthermore, a whole system requires a mechanism by which the A signal is subtracted from the A+B signal to calculate a B signal, so that there are problems of a size of circuit scale and cost. In addition, since reading out the A+B signal and the A (or B) signal is simply equivalent to doubling the number of pixels, a double readout time is required. In this way, a write time to a column circuit is increased and a horizontal transfer time is simply doubled, so that a problem that high-speed readout becomes difficult is caused.
Note that, in the case of not trying to read out both of the A+B signal and the A signal from all of the pixels but trying to read out both of the A+B signal and the A signal only from a specified row, a horizontal transfer time becomes longer only in the specified row. However, when changing a horizontal synchronizing period only in the specified row, a circuit scale of a processing circuit of a readout signal is increased and made complicated.
A technique by which, in an image capturing apparatus, a phase difference detecting function is included in an image sensor and a defocus amount of a shooting lens is thereby able to be obtained has been proposed.
For example, in Japanese Patent Laid-Open No. 2010-219958, the pupil-dividing function is provided by offsetting the sensitive region of the light-receiving portion with respect to the optical axis of the on-chip micro lens in some light-receiving elements in the image sensor. Then, by arranging pixels, which have the light-receiving elements, at predetermined intervals in the image sensor, the phase difference detecting function is realized.
Moreover, for example, in Japanese Patent Laid-Open No. 2013-106194, the plurality of photoelectric conversion elements of the A pixel and the B pixel are provided in each pixel corresponding to one of the micro lenses of the image sensor and the A pixel output and the A+B pixel output are read out. Then, by performing the subtraction processing of the A pixel output and the A+B pixel output, the B pixel output is obtained, and the phase difference detecting function is realized.